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A very unusual blood transplant appears to have cured an American man living in Berlin of infection with the AIDS virus, but doctors say the approach is not practical for wide use.

The man, who is in his 40s, had a blood stem cell transplant in 2007 to treat leukemia. His donor not only was a good blood match but also had a gene mutation that confers natural resistance to HIV.

Now, three years later, the recipient shows no signs of leukemia or HIV infection, according to a report in the journal Blood.

"It's an interesting proof-of-concept that with pretty extraordinary measures a patient could be cured of HIV," but it is far too risky to become standard therapy even if matched donors could be found, said Dr. Michael Saag of the University of Alabama at Birmingham.

He is past chairman of the HIV Medicine Association, an organization of doctors who specialize in treating AIDS.

Transplants of bone marrow — or, more commonly these days, of blood stem cells — are done to treat cancer, and their risks in healthy people is unknown.

It involves destroying the person's native immune system with powerful drugs and radiation, then replacing it with donor cells to grow a new immune system. Mortality from the procedure or its complications can be five per cent or more, Saag said.

"We can't really apply this particular approach to healthy individuals because the risk is just too high," especially when drugs can keep HIV in check in most cases, Saag said. Unless someone with HIV also had cancer, a transplant would not likely be considered, he said.

When the Berlin man's case first surfaced two years ago, Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said the procedure was too expensive and risky to be practical as a cure but that it might give more clues to using gene therapy or other methods to achieve the same result.

Dr. Julio Montaner, director of the British Columbia Centre for Excellence in HIV and AIDS, was also cautious about the findings.

There are treatments to stop HIV from progressing and stop transmission that are highly cost effective, while bone marrow transplants have high levels of mortality, Montaner said.